Fixing device and image forming apparatus

Cross-reference to related applications

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2015-023215, filed on Feb. 9, 2015, and 2015-242716, filed on Dec. 11, 2015, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.

Background

Technical Field

Exemplary aspects of the present disclosure relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.

Description of the Background

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.

Such fixing device may include a fixing rotator, such as a fixing roller, a fixing belt, and a fixing film, heated by a heater and a pressure rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotator and the pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.

Summary

This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a flexible endless belt rotatable in a predetermined direction of rotation and a pressure rotator disposed opposite the belt. A nip formation pad presses against the pressure rotator via the belt to form a fixing nip between the belt and the pressure rotator, through which a recording medium bearing a toner image is conveyed. A fixing heater is disposed opposite at least a center span of a conveyance span of the belt in an axial direction thereof where the recording medium is conveyed to heat the belt. A first lateral end heater is mounted on the nip formation pad and disposed opposite a first lateral end span of an inner circumferential surface of the belt in the axial direction thereof to heat the belt. A second lateral end heater is mounted on the nip formation pad and disposed opposite a second lateral end span of the inner circumferential surface of the belt in the axial direction thereof to heat the belt. The second lateral end heater is electrically connected in series to the first lateral end heater.

This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image bearer that bears a toner image and a fixing device disposed downstream from the image bearer in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes a flexible endless belt rotatable in a predetermined direction of rotation and a pressure rotator disposed opposite the belt. A nip formation pad presses against the pressure rotator via the belt to form a fixing nip between the belt and the pressure rotator, through which the recording medium bearing the toner image is conveyed. A fixing heater is disposed opposite at least a center span of a conveyance span of the belt in an axial direction thereof where the recording medium is conveyed to heat the belt. A first lateral end heater is mounted on the nip formation pad and disposed opposite a first lateral end span of an inner circumferential surface of the belt in the axial direction thereof to heat the belt. A second lateral end heater is mounted on the nip formation pad and disposed opposite a second lateral end span of the inner circumferential surface of the belt in the axial direction thereof to heat the belt. The second lateral end heater is electrically connected in series to the first lateral end heater.

Brief description of the drawings

A more complete appreciation of the disclosure and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present disclosure;

FIG. 2 is a schematic vertical sectional view of a fixing device incorporated in the image forming apparatus shown in FIG. 1 ;

FIG. 3 is a schematic vertical sectional view of the fixing device shown in FIG. 2 illustrating a slide sheet;

FIG. 4 is a partial vertical sectional view of the fixing device shown in FIG. 2 ;

FIG. 5 is a partial perspective view of the fixing device shown in FIG. 2 ;

FIG. 6 is a plan view of halogen heaters and lateral end heaters incorporated in the fixing device shown in FIG. 2 ;

FIG. 7 is an exploded perspective view of a nip formation assembly incorporated in the fixing device shown in FIG. 2 ;

FIG. 8 is a plan view of the lateral end heater shown in FIG. 6 ;

FIG. 9 is a schematic vertical sectional view of the fixing device illustrating the lateral end heater shown in FIG. 8 ;

FIG. 10A is a sectional view of a fixing belt, a nip formation pad, and the lateral end heaters incorporated in the fixing device shown in FIG. 2 illustrating recesses of the nip formation pad;

FIG. 10B is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters illustrating closed recesses as a first variation of the recesses shown in FIG. 10A ;

FIG. 11A is a sectional view of the nip formation pad and the lateral end heaters illustrating recesses as a second variation of the recesses shown in FIG. 10A ;

FIG. 11B is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters incorporated in the fixing device shown in FIG. 2 when a pressure roller presses the fixing belt against the lateral end heaters;

FIG. 12 is a sectional view of the fixing belt, the nip formation pad, and the lateral end heaters illustrating recesses as a third variation of the recesses shown in FIG. 10A ;

FIG. 13 is a plan view of the halogen heaters and the lateral end heaters shown in FIG. 6 illustrating a lateral end span of the lateral end heaters;

FIG. 14 is a schematic vertical sectional view of a fixing device incorporating a nip formation assembly as a variation of the nip formation assembly shown in FIG. 7 ;

FIG. 15 is a sectional view of the fixing belt incorporated in the fixing device shown in FIGS. 2, 3, 9, and 14 ;

FIG. 16 is a diagram of an electric circuit showing an electric connection between the halogen heaters and the lateral end heaters of the fixing device shown in FIGS. 2, 9, and 14 according to a first exemplary embodiment of the present disclosure;

FIG. 17 is a diagram of an electric circuit showing the electric connection between the halogen heaters and the lateral end heaters of the fixing device shown in FIGS. 2, 9, and 14 according to a second exemplary embodiment of the present disclosure;

FIG. 18 is a graph showing a relation between time and a temperature of the fixing belt of the fixing device shown in FIG. 2 ; and

FIG. 19 is a diagram of an electric circuit according to a third exemplary embodiment of the present disclosure.

Detailed description of the disclosure

In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to FIG. 1 , an image forming apparatus 100 according to an exemplary embodiment of the present disclosure is explained.

It is to be noted that, in the drawings for explaining exemplary embodiments of this disclosure, identical reference numerals are assigned, as long as discrimination is possible, to components such as members and component parts having an identical function or shape, thus omitting description thereof once it is provided.

FIG. 1 is a schematic vertical sectional view of the image forming apparatus 100 . The image forming apparatus 100 may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this exemplary embodiment, the image forming apparatus 100 is a color printer that forms color and monochrome toner images on a recording medium by electrophotography. Alternatively, the image forming apparatus 100 may be a monochrome printer that forms a monochrome toner image on a recording medium.

A description is provided of a construction and an operation of the image forming apparatus 100 .

The image forming apparatus 100 is a color printer employing a tandem system in which a plurality of image forming devices for forming toner images in a plurality of colors, respectively, is aligned in a rotation direction of an intermediate transfer belt.

The image forming apparatus 100 includes four photoconductive drums 20 Y, 20 C, 20 M, and 20 K serving as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively, that is, yellow, cyan, magenta, and black. The yellow, cyan, magenta, and black toner images formed on the photoconductive drums 20 Y, 20 C, 20 M, and 20 K as visible images, respectively, are primarily transferred successively onto an intermediate transfer belt 11 serving as an intermediate transferor disposed opposite the photoconductive drums 20 Y, 20 C, 20 M, and 20 K as the intermediate transfer belt 11 rotates in a rotation direction A 1 such that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt 11 in a primary transfer process. Thereafter, the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt 11 are secondarily transferred onto a sheet S serving as a recording medium collectively in a secondary transfer process. Each of the photoconductive drums 20 Y, 20 C, 20 M, and 20 K is surrounded by image forming components that form the yellow, cyan, magenta, and black toner images on the photoconductive drums 20 Y, 20 C, 20 M, and 20 K as they rotate clockwise in FIG. 1 in a rotation direction D 20 .

Taking the photoconductive drum 20 K that forms the black toner image, the following describes a construction of components that form the black toner image. The photoconductive drum 20 K is surrounded by a charger 30 K, a developing device 40 K, a primary transfer roller 12 K, and a cleaner 50 K in this order in the rotation direction D 20 of the photoconductive drum 20 K. The photoconductive drums 20 Y, 20 C, and 20 M are also surrounded by chargers 30 Y, 30 C, and 30 M, developing devices 40 Y, 40 C, and 40 M, primary transfer rollers 12 Y, 12 C, and 12 M, and cleaners 50 Y, 50 C, and 50 M in this order in the rotation direction D 20 of the photoconductive drums 20 Y, 20 C, and 20 M, respectively. The charger 30 K uniformly changes an outer circumferential surface of the photoconductive drum 20 K. An optical writing device 8 optically writes an electrostatic latent image on the charged outer circumferential surface of the photoconductive drum 20 K according to image data sent from an external device such as a client computer. The developing device 40 K visualizes the electrostatic latent image as a black toner image.

As the intermediate transfer belt 11 rotates in the rotation direction A 1 , the yellow, cyan, magenta, and black toner images formed on the photoconductive drums 20 Y, 20 C, 20 M, and 20 K, respectively, are primarily transferred successively onto the intermediate transfer belt 11 , thus being superimposed on the same position on the intermediate transfer belt 11 and formed into a color toner image. In the primary transfer process, the primary transfer rollers 12 Y, 12 C, 12 M, and 12 K disposed opposite the photoconductive drums 20 Y, 20 C, 20 M, and 20 K via the intermediate transfer belt 11 , respectively, apply a primary transfer bias to the photoconductive drums 20 Y, 20 C, 20 M, and 20 K successively from the upstream photoconductive drum 20 Y to the downstream photoconductive drum 20 K in the rotation direction A 1 of the intermediate transfer belt 11 . The photoconductive drums 20 Y, 20 C, 20 M, and 20 K are aligned in this order in the rotation direction A 1 of the intermediate transfer belt 11 . The photoconductive drums 20 Y, 20 C, 20 M, and 20 K are located in four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively.

The image forming apparatus 100 includes the four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively, an intermediate transfer belt unit 10 , a secondary transfer roller 5 , an intermediate transfer belt cleaner 13 , and the optical writing device 8 . The intermediate transfer belt unit 10 is situated above and disposed opposite the photoconductive drums 20 Y, 20 C, 20 M, and 20 K. The intermediate transfer belt unit 10 incorporates the intermediate transfer belt 11 and the primary transfer rollers 12 Y, 12 C, 12 M, and 12 K. The secondary transfer roller 5 serves as a secondary transferor disposed opposite the intermediate transfer belt 11 and driven and rotated in accordance with rotation of the intermediate transfer belt 11 . The intermediate transfer belt cleaner 13 is disposed opposite the intermediate transfer belt 11 to clean the intermediate transfer belt 11 . The optical writing device 8 is situated below and disposed opposite the four image forming stations.

The optical writing device 8 includes a semiconductor laser serving as a light source, a coupling lens, an fθ lens, a troidal lens, a deflection mirror, and a rotatable polygon mirror serving as a deflector. The optical writing device 8 emits light beams Lb corresponding to the yellow, cyan, magenta, and black toner images to be formed on the photoconductive drums 20 Y, 20 C, 20 M, and 20 K thereto, forming electrostatic latent images on the photoconductive drums 20 Y, 20 C, 20 M, and 20 K, respectively. FIG. 1 illustrates the light beam Lb irradiating the photoconductive drum 20 K. Similarly, light beams irradiate the photoconductive drums 20 Y, 20 C, and 20 M, respectively.

The image forming apparatus 100 further includes a sheet feeder 61 and a registration roller pair 4 . The sheet feeder 61 , disposed in a lower portion of the image forming apparatus 100 , incorporates a paper tray that loads a plurality of sheets S to be conveyed to a secondary transfer nip formed between the intermediate transfer belt 11 and the secondary transfer roller 5 . The registration roller pair 4 serving as a conveyor conveys the sheet S conveyed from the sheet feeder 61 to the secondary transfer nip formed between the intermediate transfer belt 11 and the secondary transfer roller 5 at a predetermined time when the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt 11 reach the secondary transfer nip. The image forming apparatus 100 further includes a sensor for detecting that a leading edge of the sheet S reaches the registration roller pair 4 .

The secondary transfer roller 5 secondarily transfers the color toner image formed on the intermediate transfer belt 11 onto the sheet S as the sheet S is conveyed through the secondary transfer nip. The sheet S bearing the color toner image is conveyed to a fixing device 150 where the color toner image is fixed on the sheet S under heat and pressure. An output roller pair 7 ejects the sheet S bearing the fixed color toner image onto an output tray disposed atop the image forming apparatus 100 . In an upper portion of the image forming apparatus 100 and below the output tray are toner bottles 9 Y, 9 C, 9 M, and 9 K containing fresh yellow, cyan, magenta, and black toners, respectively.

The intermediate transfer belt unit 10 includes a driving roller 72 and a driven roller 73 over which the intermediate transfer belt 11 is looped, in addition to the intermediate transfer belt 11 and the primary transfer rollers 12 Y, 12 C, 12 M, and 12 K. Since the driven roller 73 also serves as a tension applicator that applies tension to the intermediate transfer belt 11 , a biasing member (e.g., a spring) biases the driven roller 73 against the intermediate transfer belt 11 . The intermediate transfer belt unit 10 , the primary transfer rollers 12 Y, 12 C, 12 M, and 12 K, the secondary transfer roller 5 , and the intermediate transfer belt cleaner 13 constitute a transfer device 71 . The sheet feeder 61 includes a feed roller 3 that contacts an upper side of an uppermost sheet S of the plurality of sheets S loaded on the paper tray of the sheet feeder 61 . As the feed roller 3 is driven and rotated counterclockwise in FIG. 1 , the feed roller 3 feeds the uppermost sheet S to the registration roller pair 4 .

The intermediate transfer belt cleaner 13 of the transfer device 71 includes a cleaning brush and a cleaning blade disposed opposite the intermediate transfer belt 11 to come into contact with the intermediate transfer belt 11 . The cleaning brush and the cleaning blade scrape a foreign substance such as residual toner particles off the intermediate transfer belt 11 , removing the foreign substance from the intermediate transfer belt 11 and thereby cleaning the intermediate transfer belt 11 . The intermediate transfer belt cleaner 13 further includes a waste toner conveyer that conveys the residual toner particles removed from the intermediate transfer belt 11 .

With reference to FIG. 2 , a description is provided of a configuration of the fixing device 150 incorporated in the image forming apparatus 100 described above.

FIG. 2 is a schematic vertical sectional view of the fixing device 150 . As shown in FIG. 2 , the fixing device 150 (e.g., a fuser or a fusing unit) includes a thin, flexible, endless fixing belt 80 , serving as an endless belt or a fixing rotator, formed into a loop and rotatable in a rotation direction D 80 and a pressure roller 84 serving as a pressure rotator disposed opposite the fixing belt 80 and rotatable in a rotation direction D 84 . Inside the loop formed by the fixing belt 80 is a nip formation assembly 86 (e.g., a nip formation unit) that forms a fixing nip N between the fixing belt 80 and the pressure roller 84 , through which the sheet S is conveyed.

A detailed description is now given of a construction of the nip formation assembly 86 .

The nip formation assembly 86 includes a nip formation pad 88 , a lateral end heater 112 , and a stay 90 . The nip formation pad 88 , disposed inside the loop formed by the fixing belt 80 and disposed opposite the pressure roller 84 , presses against the pressure roller 84 via the fixing belt 80 to form the fixing nip N between the fixing belt 80 and the pressure roller 84 . The lateral end heater 112 serving as a lateral end heater or a lateral end heat source is mounted on each lateral end of the nip formation pad 88 in a longitudinal direction thereof parallel to an axial direction of the fixing belt 80 . The stay 90 supports the nip formation pad 88 against pressure from the pressure roller 84 .

FIG. 3 is a schematic vertical sectional view of the fixing device 150 illustrating a slide sheet. As shown in FIG. 3 , an inner circumferential surface of the fixing belt 80 slides over the nip formation pad 88 via a low-friction sheet 6 serving as a slide sheet. The low-friction sheet 6 is applied with a lubricant such as fluorine grease and silicone oil to decrease a slide torque of the fixing belt 80 . Alternatively, the nip formation pad 88 may contact the inner circumferential surface of the fixing belt 80 directly without the low-friction sheet 6 sandwiched between the nip formation pad 88 and the fixing belt 80 .

The stay 90 has a box shape with an opening opposite the fixing nip N. Two halogen heaters 82 a and 82 b serving as a fixing heater or a fixing heat source are disposed inside the box of the stay 90 . The halogen heaters 82 a and 82 b emit light that irradiates the inner circumferential surface of the fixing belt 80 directly through the opening of the stay 90 , heating the fixing belt 80 with radiation heat. A platy reflector 94 is mounted on an interior surface of the stay 90 to reflect light radiated from the halogen heaters 82 a and 82 b toward the fixing belt 80 so as to improve heating efficiency of the halogen heaters 82 a and 82 b to heat the fixing belt 80 . The reflector 94 prevents light from the halogen heaters 82 a and 82 b from heating the stay 90 , suppressing waste of energy. Alternatively, instead of the reflector 94 , the interior surface of the stay 90 may be treated with insulation or mirror finish to reflect light radiated from the halogen heaters 82 a and 82 b toward the fixing belt 80 .

A detailed description is now given of a construction of the pressure roller 84 .

FIG. 4 is a partial vertical sectional view of the fixing device 150 . As shown in FIG. 4 , the pressure roller 84 is constructed of a hollow metal roller 84 a , an elastic layer 84 b coating an outer circumferential surface of the metal roller 84 a and being made of silicone rubber, and a release layer 84 c coating an outer circumferential surface of the elastic layer 84 b . The release layer 84 c , having a layer thickness in a range of from 5 micrometers to 50 micrometers, is made of perfluoroalkoxy fluoro resin (PFA) or polytetrafluoroethylene (PTFE) to facilitate separation of the sheet S from the pressure roller 84 . As a driving force generated by a driver (e.g., a motor) situated inside the image forming apparatus 100 depicted in FIG. 1 is transmitted to the pressure roller 84 through a gear train, the pressure roller 84 rotates in the rotation direction D 84 as shown in FIG. 2 . Alternatively, the driver may also be connected to the fixing belt 80 to drive and rotate the fixing belt 80 . A spring or the like biases the pressure roller 84 against the fixing belt 80 . As the elastic layer 84 b of the pressure roller 84 is pressed and deformed, the pressure roller 84 produces the fixing nip N having a predetermined length Nw in a sheet conveyance direction DS as shown in FIG. 4 . Alternatively, the pressure roller 84 may be a solid roller. However, a hollow roller has a decreased thermal capacity. Further, a heater or a heat source such as a halogen heater may be disposed inside the pressure roller 84 . The elastic layer 84 b may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller 84 , the elastic layer 84 b may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because the sponge rubber has an increased insulation that draws less heat from the fixing belt 80 .

A detailed description is now given of a construction of the fixing belt 80 .

The fixing belt 80 is an endless belt or film having a layer thickness in a range of from 30 micrometers to 50 micrometers and made of metal such as nickel and SUS stainless steel or resin such as polyimide. The fixing belt 80 is constructed of a base layer and a release layer. The release layer constituting an outer surface layer is made of PFA, PTFE, or the like to facilitate separation of toner of a toner image on the sheet S from the fixing belt 80 , thus preventing the toner of the toner image from adhering to the fixing belt 80 . Optionally, an elastic layer may be sandwiched between the base layer and the release layer and made of silicone rubber or the like. If the fixing belt 80 does not incorporate the elastic layer, the fixing belt 80 has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image is fixed on the sheet S. However, as the pressure roller 84 and the fixing belt 80 sandwich and press the unfixed toner image on the sheet S passing through the fixing nip N, slight surface asperities of the fixing belt 80 may be transferred onto the toner image on the sheet S, resulting in variation in gloss of the solid toner image on the sheet S.

To address this circumstance, the elastic layer made of silicone rubber has a thickness not smaller than 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing belt 80 , suppressing variation in gloss of the toner image on the sheet S. As shown in FIG. 2 , as the pressure roller 84 rotates in the rotation direction D 84 , the fixing belt 80 rotates in the rotation direction D 80 in accordance with rotation of the pressure roller 84 by friction therebetween. At the fixing nip N, the fixing belt 80 rotates as it is sandwiched between the pressure roller 84 and the nip formation pad 88 ; at a circumferential span of the fixing belt 80 other than the fixing nip N, the fixing belt 80 rotates while the fixing belt 80 is supported at each lateral end in the axial direction thereof to retain a tubular shape. Thus, the fixing belt 80 is retained circular in cross-section stably. As shown in FIG. 2 , a separator 32 is disposed downstream from the fixing nip N in the sheet conveyance direction DS to separate the sheet S from the fixing belt 80 .

According to this exemplary embodiment, as shown in FIGS. 2 to 4 , the fixing nip N is planar. Alternatively, the fixing nip N may define a curve projecting toward the fixing belt 80 to produce a recess in the fixing belt 80 in cross-section or other shapes. If the fixing nip N defines the recess in the fixing belt 80 , the recessed fixing nip N directs the leading edge of the sheet S toward the pressure roller 84 as the sheet S is ejected from the fixing nip N, facilitating separation of the sheet S from the fixing belt 80 and suppressing jamming of the sheet S. In this case, a nip formation face of the nip formation pad 88 is contoured into the recess. Similarly, a fixing belt side face of the lateral end heater 112 coupled with the nip formation pad 88 may be contoured along the recessed nip formation face of the nip formation pad 88 .

A detailed description is now given of a configuration of the stay 90 .

The stay 90 supports the nip formation pad 88 against pressure from the pressure roller 84 to prevent bending of the nip formation pad 88 and produce the even length Nw of the fixing nip N in the sheet conveyance direction DS throughout the entire width of the fixing belt 80 in the axial direction thereof. According to this exemplary embodiment, the pressure roller 84 is pressed against the fixing belt 80 to form the fixing nip N. Alternatively, the nip formation assembly 86 may be pressed against the pressure roller 84 to from the fixing nip N. The stay 90 has a mechanical strength great enough to support the nip formation pad 88 to prevent bending of the nip formation pad 88 . The stay 90 is made of metal such as stainless steel and iron, metallic oxide such as ceramic, or the like. The fixing belt 80 and the components disposed inside the loop formed by the fixing belt 80 , that is, the halogen heaters 82 a and 82 b , the nip formation pad 88 , the lateral end heater 112 , the stay 90 , and the reflector 94 , may constitute a belt unit 80 U separably coupled with the pressure roller 84 .

FIG. 5 is a partial perspective view of the fixing device 150 . As shown in FIG. 5 , both lateral ends of the fixing belt 80 in the axial direction thereof are rotatably supported by flanges 36 , respectively. Each of the flanges 36 serves as a support projecting from a side plate 34 in the axial direction of the fixing belt 80 . Although FIG. 5 illustrates the flange 36 and the side plate 34 situated at one lateral end of the fixing belt 80 in the axial direction thereof, the flange 36 and the side plate 34 are also situated at another lateral end of the fixing belt 80 in the axial direction thereof. The flange 36 that guides each lateral end of the fixing belt 80 in the axial direction thereof has an outer diameter substantially equivalent to an inner diameter of the fixing belt 80 . The flange 36 projects inboard from a lateral edge of the fixing belt 80 by a length in a range of from 5 mm to 10 mm in the axial direction of the fixing belt 80 . The flanges 36 guide the fixing belt 80 even when the fixing belt 80 rotates, retaining the fixing belt 80 to be circular in cross-section. The flange 36 includes a slit 36 a disposed opposite the fixing nip N to place the nip formation assembly 86 at a predetermined position. The stay 90 depicted in FIG. 2 has a width that spans the entire width of the fixing belt 80 in the axial direction thereof. Both lateral ends of the stay 90 in the axial direction of the fixing belt 80 are fixedly mounted on the side plates 34 , respectively, thus being supported and positioned by the side plates 34 .

FIG. 6 is a plan view of the halogen heaters 82 a and 82 b illustrating a light distribution of the halogen heaters 82 a and 82 b and a positional relation between the halogen heaters 82 a and 82 b and a plurality of lateral end heaters 112 a and 112 b . FIG. 6 illustrates a width W 1 of an A3 size sheet in portrait orientation in the axial direction of the fixing belt 80 and a width W 2 of an A3 extension size sheet and a 13-inch size sheet in portrait orientation in the axial direction of the fixing belt 80 . As shown in FIG. 6 , the halogen heater 82 a is a center heater having a dense light distribution at a center span of the halogen heater 82 a disposed opposite a center span of the fixing belt 80 in the axial direction thereof where a small sheet S having a decreased width in the axial direction of the fixing belt 80 is conveyed over the fixing belt 80 . Conversely, the halogen heater 82 b is an outboard heater having a dense light distribution at each lateral end span of the halogen heater 82 b disposed outboard from the halogen heater 82 a and disposed opposite each lateral end span of the fixing belt 80 in the axial direction thereof where a medium sheet S having a medium width (e.g., an A3 size sheet) in the axial direction of the fixing belt 80 is conveyed over the fixing belt 80 . As the small sheet S is conveyed over the fixing belt 80 , the halogen heater 82 a is powered on and the halogen heater 82 b is not powered on, thus preventing each lateral end span of the fixing belt 80 in the axial direction thereof where the small sheet S is not conveyed from being heated unnecessarily.

A description is provided of a configuration of a first comparative fixing device incorporating a center heater equivalent to the halogen heater 82 a and a lateral end heater equivalent to the halogen heater 82 b depicted in FIG. 6 .

As a small sheet S is conveyed through the fixing nip N, the center heater having a center dense light distribution is powered on. As a medium sheet S is conveyed through the fixing nip N, the lateral end heater having a lateral end dense light distribution is powered on together with the center heater. The center heater and the lateral end heater are powered on and off properly to heat sheets S of various sizes.

Taking the sizes of the sheets S and the frequency with which the sheets S are conveyed, sheets S up to the A3 size sheet are used frequently. The A3 size sheet is conveyed through the fixing nip N in portrait orientation. An A4 size sheet and a letter (LT) size sheet that are used with an increased frequency are generally conveyed in landscape orientation to enhance productivity. To address this circumstance, the center heater and the lateral end heater produce a heating span of about 300 mm in the axial direction of the fixing belt 80 that is great enough to heat 99 percent or more of the sizes of sheets S. On the other hand, the center heater and the lateral end heater are requested to heat large sheets S greater than the A3 size sheet in the axial direction of the fixing belt 80 such as the A3 extension size sheet and the 13-inch sheet although the large sheets S are used infrequently.

If a plurality of halogen heaters is used as the center heater and the lateral end heater, respectively, the plurality of halogen heaters used to heat the small sheet S is situated inside the loop formed by the fixing belt 80 or a fixing roller having a diameter of about 30 mm. Accordingly, the number of the halogen heaters is limited. To address this circumstance, the lateral end heater having the lateral end dense light distribution may be elongated to span a width of the large sheet S greater than a width of the A3 size sheet in the axial direction of the fixing belt 80 . As described above, the center heater and the lateral end heater heat the heating span of about 300 mm of the fixing belt 80 in the axial direction thereof frequently. However, if the elongated lateral end heater is employed, the elongated lateral end heater may heat an elongated heating span of about 330 mm of the fixing belt 80 in the axial direction thereof, wasting energy used to heat a differential between the heating span of about 300 mm and the elongated heating span of about 330 mm. When the A3 size sheet in portrait orientation or the A4 size sheet in landscape orientation is conveyed through the fixing nip N, each lateral end of the elongated heating span of the fixing belt 80 in the axial direction thereof that corresponds to the differential between the heating span of about 300 mm and the elongated heating span of about 330 mm may overheat. In order to cool the overheated lateral end of the fixing belt 80 , productivity defined by a conveyance speed of the sheets S may be degraded or a fan may be installed. If a reflection plate is interposed between the lateral end heater and the fixing belt 80 , each lateral end of the lateral end heater in the axial direction of the fixing belt 80 may overheat. To address this circumstance, a second comparative fixing device is proposed.

The second comparative fixing device includes a thin, flexible endless belt to be heated quickly to a fixing temperature at which a toner image is fixed on a sheet S and a nip formation unit located inside a loop formed by the endless belt. The nip formation unit presses against a pressure roller via the endless belt to form a fixing nip between the endless belt and the pressure roller. A plurality of halogen heaters having different light distributions, respectively, is situated inside the loop formed by the endless belt. A plurality of lateral end heaters is disposed opposite both lateral end spans of the endless belt in an axial direction thereof, respectively, and upstream from the fixing nip in a rotation direction of the endless belt so as to heat an increased heating span of the endless belt corresponding to the width of the large sheet S in the axial direction of the endless belt. The lateral end heaters contact an inner circumferential surface or an outer circumferential surface of the endless belt. The lateral end heaters heat the increased heating span of the endless belt corresponding to the width of the large sheet S in the axial direction of the endless belt with a simple construction not incorporating an extra halogen heater directed to the large sheet S.

The lateral end heaters are disposed opposite both lateral end spans of the endless belt in the axial direction thereof, respectively. Accordingly, the lateral end heaters are powered on and off simultaneously, complicating a control circuit that controls the lateral end heaters. Even if one of the lateral end heaters suffers from failure, another one of the lateral end heaters is energized and is susceptible to electric failure.

A width of the A3 size sheet in portrait orientation and a width of the A4 size sheet in landscape orientation are smaller than a width of the A3 extension size sheet in portrait orientation (e.g., 329 mm) and a width of the 13-inch sheet in portrait orientation (e.g., 330 mm) by a differential in a range of from 32 mm to 33 mm, respectively. Accordingly, if the fixing device 150 is configured to heat each lateral end span of the fixing belt 80 in the axial direction thereof, that is, if the fixing device 150 is configured to heat a half of the differential in range of from 32 mm to 33 mm, that is, a span in a range of from 16.0 mm to 16.5 mm, the maximum width of sheets S available in the fixing device 150 increases from the width W 1 of the A3 size sheet to the width W 2 of the A3 extension size sheet or the like as shown in FIG. 6 . In other words, if the fixing device 150 is configured to heat each outboard span, that is, each lateral end, of the fixing belt 80 disposed opposite an outboard span of the halogen heater 82 b that is outboard from each lateral end span of the halogen heater 82 b in the axial direction of the fixing belt 80 and does not have the dense light distribution, the large sheet S (e.g., the A3 extension size sheet) is available in the fixing device 150 . Accordingly, the fixing device 150 includes the lateral end heater 112 constructed of downsized heaters, that is, the lateral end heaters 112 a and 112 b having a decreased width of about 20 mm in the axial direction of the fixing belt 80 .

As the large sheet S (e.g., the A3 extension size sheet and the 13-inch sheet) is conveyed through the fixing nip N, the halogen heaters 82 a and 82 b and the lateral end heaters 112 a and 112 b are energized. Conversely, as the small sheet S (e.g., a sheet not greater than the A3 size sheet) is conveyed through the fixing nip N, the halogen heaters 82 a and 82 b are energized or the halogen heater 82 a is energized. Hence, the lateral end heaters 112 a and 112 b are not energized. If the halogen heater 82 b is configured to have an increased heating span to heat the large sheet S such as the A3 extension size sheet, the halogen heater 82 b may heat the lateral end span of the fixing belt 80 unnecessarily while the large sheet S is not conveyed through the fixing nip N, wasting energy. To address this circumstance, the fixing device 150 according to this exemplary embodiment incorporates a simple mechanism in addition to the halogen heaters 82 a and 82 b , that is, the lateral end heaters 112 a and 112 b being disposed opposite both lateral end spans G in the axial direction of the fixing belt 80 or in proximity to both lateral ends of the fixing belt 80 in the axial direction thereof, respectively.

The description continues in the full USPTO document.